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Re: [PATCH 1/2] Add a fast path for C rd/wrlock


On Tue, 2014-03-25 at 16:24 -0700, Andi Kleen wrote:
> From: Andi Kleen <ak@linux.intel.com>
> 
> One difference of the C versions to the assembler wr/rdlock
> is that the C compiler saves some registers which are unnecessary
> for the fast path in the prologue of the functions. Split the
> uncontended fast path out into a separate function. Only when contention is
> detected is the full featured function called. This makes
> the fast path code (nearly) identical to the assembler version,
> and gives uncontended performance within a few cycles.
> 
> nptl/:
> 2014-03-25  Andi Kleen  <ak@linux.intel.com>
> 
> 	* pthread_rwlock_rdlock (__pthread_rwlock_rdlock):
> 	Split into __do_pthread_rwlock_rdlock and __pthread_rwlock_rdlock.
> 	* pthread_rwlock_wrlock (__pthread_rwlock_wrlock):
> 	Split into __do_pthread_rwlock_wrlock and __pthread_wrlock_rdlock.
> ---
>  nptl/pthread_rwlock_rdlock.c | 88 ++++++++++++++++++++++++++++++--------------
>  nptl/pthread_rwlock_wrlock.c | 59 ++++++++++++++++++++---------
>  2 files changed, 103 insertions(+), 44 deletions(-)
> 
> diff --git a/nptl/pthread_rwlock_rdlock.c b/nptl/pthread_rwlock_rdlock.c
> index 3773f7d..a4deed4 100644
> --- a/nptl/pthread_rwlock_rdlock.c
> +++ b/nptl/pthread_rwlock_rdlock.c
> @@ -24,39 +24,16 @@
>  #include <stap-probe.h>
>  
> 
> -/* Acquire read lock for RWLOCK.  */
> -int
> -__pthread_rwlock_rdlock (rwlock)
> -     pthread_rwlock_t *rwlock;
> +/* Acquire read lock for RWLOCK.  Slow path. */

Double space before end of comment.

> +static int __attribute__((noinline))
> +__do_pthread_rwlock_rdlock (pthread_rwlock_t *rwlock)

I'd prefer renaming that to __pthread_rwlock_rdlock_slow.
Alternatively, we could use the "_full" suffix, as the mutex code is
doing.

>  {
>    int result = 0;
>  
> -  LIBC_PROBE (rdlock_entry, 1, rwlock);
> -
> -  /* Make sure we are alone.  */
> -  lll_lock (rwlock->__data.__lock, rwlock->__data.__shared);
> +  /* Lock is taken in caller. */

Double space before end of comment.

>  
>    while (1)
>      {
> -      /* Get the rwlock if there is no writer...  */
> -      if (rwlock->__data.__writer == 0
> -	  /* ...and if either no writer is waiting or we prefer readers.  */
> -	  && (!rwlock->__data.__nr_writers_queued
> -	      || PTHREAD_RWLOCK_PREFER_READER_P (rwlock)))
> -	{
> -	  /* Increment the reader counter.  Avoid overflow.  */
> -	  if (__glibc_unlikely (++rwlock->__data.__nr_readers == 0))
> -	    {
> -	      /* Overflow on number of readers.	 */
> -	      --rwlock->__data.__nr_readers;
> -	      result = EAGAIN;
> -	    }
> -	  else
> -	    LIBC_PROBE (rdlock_acquire_read, 1, rwlock);
> -
> -	  break;
> -	}
> -
>        /* Make sure we are not holding the rwlock as a writer.  This is
>  	 a deadlock situation we recognize and report.  */
>        if (__builtin_expect (rwlock->__data.__writer
> @@ -88,6 +65,25 @@ __pthread_rwlock_rdlock (rwlock)
>        lll_lock (rwlock->__data.__lock, rwlock->__data.__shared);
>  
>        --rwlock->__data.__nr_readers_queued;
> +
> +      /* Get the rwlock if there is no writer...  */
> +      if (rwlock->__data.__writer == 0
> +	  /* ...and if either no writer is waiting or we prefer readers.  */
> +	  && (!rwlock->__data.__nr_writers_queued
> +	      || PTHREAD_RWLOCK_PREFER_READER_P (rwlock)))
> +	{
> +	  /* Increment the reader counter.  Avoid overflow.  */
> +	  if (__glibc_unlikely (++rwlock->__data.__nr_readers == 0))
> +	    {
> +	      /* Overflow on number of readers.	 */
> +	      --rwlock->__data.__nr_readers;
> +	      result = EAGAIN;
> +	    }
> +	  else
> +	    LIBC_PROBE (rdlock_acquire_read, 1, rwlock);
> +
> +	  break;
> +	}
>      }
>  
>    /* We are done, free the lock.  */
> @@ -96,5 +92,43 @@ __pthread_rwlock_rdlock (rwlock)
>    return result;
>  }
>  
> +
> +/* Fast path of acquiring read lock on RWLOCK.  */
> +
> +int
> +__pthread_rwlock_rdlock (pthread_rwlock_t *rwlock)
> +{
> +  int result = 0;
> +
> +  LIBC_PROBE (rdlock_entry, 1, rwlock);
> +
> +  /* Make sure we are alone.  */
> +  lll_lock (rwlock->__data.__lock, rwlock->__data.__shared);
> +
> +  /* Get the rwlock if there is no writer...  */
> +  if (rwlock->__data.__writer == 0
> +      /* ...and if either no writer is waiting or we prefer readers.  */
> +      && (!rwlock->__data.__nr_writers_queued
> +	  || PTHREAD_RWLOCK_PREFER_READER_P (rwlock)))
> +    {
> +      /* Increment the reader counter.  Avoid overflow.  */
> +      if (__glibc_unlikely (++rwlock->__data.__nr_readers == 0))
> +	{
> +	  /* Overflow on number of readers.	 */
> +	  --rwlock->__data.__nr_readers;
> +	  result = EAGAIN;
> +	}
> +      else
> +	LIBC_PROBE (rdlock_acquire_read, 1, rwlock);
> +
> +      /* We are done, free the lock.  */
> +      lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);
> +
> +      return result;
> +    }
> +
> +  return __do_pthread_rwlock_rdlock (rwlock);
> +}
> +
>  weak_alias (__pthread_rwlock_rdlock, pthread_rwlock_rdlock)
>  hidden_def (__pthread_rwlock_rdlock)
> diff --git a/nptl/pthread_rwlock_wrlock.c b/nptl/pthread_rwlock_wrlock.c
> index 1613d45..2907681 100644
> --- a/nptl/pthread_rwlock_wrlock.c
> +++ b/nptl/pthread_rwlock_wrlock.c
> @@ -25,29 +25,15 @@
>  
> 
>  /* Acquire write lock for RWLOCK.  */
> -int
> -__pthread_rwlock_wrlock (rwlock)
> -     pthread_rwlock_t *rwlock;
> +static int __attribute__((noinline))
> +__do_pthread_rwlock_wrlock (pthread_rwlock_t *rwlock)

See above.



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